The present invention relates to a method for preparing a glass-ceramic. The pulverized glass-ceramics (glass-ceramic powders) obtained by the method of the invention is useful particularly as a porcelain to be built up and fused for coating the surface of a metal frame and thereby fabricating a dental prosthesis with excellent aesthetic qualities.
A technique is known in which a porcelain powder (hereinafter referred to simply as xe2x80x9cporcelainxe2x80x9d) comprising a glass-ceramic is built up on and fusion-bonded to the surface of a metal frame to fabricate a dental prosthesis which has an appearance similar to natural teeth and high mechanical and chemical durability. The metal frame is mainly made of a precious metal alloy which has an approximately constant coefficient of thermal expansion (about (14.2xc2x10.5)xc3x9710xe2x88x926/xc2x0 C.).
In such dental prosthesis fabrication, a coating process comprising building up, fusion-bonding and cooling a porcelain on the metal frame surface is repeated several times to form the external shape of the dental prosthesis. Stated more specifically, the fabrication includes the basic steps of forming three layers by sequentially building up and fusion-bonding an undercoat opaque for concealment of the metal color and fusion bonding, a dentine porcelain that determines the basic color of the prosthetic teeth, and an enamel porcelain that reproduces the characteristics of tooth enamel. In addition, the fabrication involves the step of forming a margin that matches with natural teeth, and adjusting steps such as coloring, color tone modification and the like. Thus, in the known technique, the coating process comprising building up, fusion-bonding and cooling a porcelain on a metal frame is repeated at least 3 times, up to about 10 times.
Therefore, the porcelains are required to have a coefficient of thermal expansion approximating that of the material of the metal frame and have thermal stability such that their coefficients of thermal expansion scarcely change during the repeated coating process.
Leucite crystals are represented by the chemical formula 4SiO2.Al2O3.K2O(xe2x95x90KAlSi2O6). Since leucite crystals have a large coefficient of thermal expansion, a glass-ceramic containing a specific amount of leucite crystals has a coefficient of thermal expansion approximately equal to that of the material of the metal frame. Further, when the leucite crystal phase coexists with a glass matrix phase, the two phases as a whole have uniform light transmittance (transparency) because the refractive indices of the two phases are close to each other. Therefore, addition of a coloring ingredient to a porcelain comprising the coexisting mixture enables desired coloration of a restored dental prosthesis to impart a highly aesthetic appearance similar to the appearance of natural teeth. Because of these excellent characteristics (coefficient of thermal expansion and transparency) of leucite crystal-containing glass-ceramics, the use of leucite crystal-containing glass-ceramics as materials (porcelains) for coating metal frames has been proposed.
For example, U.S. Pat. No. 4,604,366 discloses a method for preparing a ceramic porcelain, comprising blending a matrix glass with several types of leucite crystal-containing glass-ceramic frits having different leucite crystal contents and different coefficients of thermal expansion. However, this method necessitates troublesome procedures for determining the ratio of the at least three types of ingredients and blending these ingredients, in order to suitably control the leucite crystal content and coefficient of thermal expansion of the final product porcelain. Moreover, the dental prosthesis obtained by firing the ceramic porcelain has the serious problem of non-uniform leucite crystal distribution.
U.S. Pat. No. 4,798,536 discloses a method for preparing a ceramic porcelain, comprising mixing a natural feldspar, such as Wyoming feldspar, as a leucite crystal origin point with a glass matrix-forming ingredient, and melting, slowly cooling and then suddenly cooling the mixture. However, this method includes troublesome procedures for purifying the natural feldspar, and is complicated as a whole. In addition, trace impurities derived from the natural feldspar are liable to remain in the porcelain and decrease the transparency, thereby deteriorating the color of the resulting dental prosthesis.
When a porcelain is fusion-bonded to a metal frame, a lower fusion-bonding temperature is desirable from a workability point of view. However, the leucite crystal phase is instable in xe2x80x9clow-fusing porcelainxe2x80x9d which is softened and fluidized at 750 to 950xc2x0 C. and fusion-bonded to a metal surface. Thus, if the porcelain is maintained at 950xc2x0 C. or lower, the leucite crystal phase undergoes transformation to a different type of crystal phase, or shifts to a state of coexistence with a different type of crystal phase. More specifically, when a powder of a known low-fusing, leucite crystal-containing glass-ceramic (a composite of leucite crystals and matrix phase) is fired at 750 to 950xc2x0 C. to coat a metal frame, Naxe2x80x94K feldspathic crystals (high temperature-type Naxe2x80x94K sanidine) start to precipitate after a given period of time, and then the leucite crystals begin to decrease and finally disappear, since the leucite crystals contained are a metastable crystal phase. The precipitation of the Naxe2x80x94K feldspathic crystals lowers the coefficient of thermal expansion and causes opacification in the glass-ceramic. Thus, the coefficient of thermal expansion of the glass-ceramic gradually decreases during the repeated coating process comprising building up, fusion-bonding and cooling of the glass-ceramic on a metal frame. As a result, the glass-ceramic has defects such as cracks owing to the stress of strain between the frame material and the ceramic coating layer, leading to low adhesion between the frame material and the ceramic coating layer. Further, the opacification of the glass-ceramic impairs the transparency of the ceramic coating layer.
In the above situation, the development of a novel porcelain which can be easily prepared and is free of deterioration in characteristics (i.e., decrease in the coefficient of thermal expansion, or opacification) during a process of coating a metal frame is desired.
An object of the present invention is to provide a leucite crystal-containing glass-ceramic which can be easily prepared.
Another object of the invention is to provide a leucite crystal-containing glass-ceramic whose leucite crystal content does not substantially change when heated, and which exhibits a stable coefficient of thermal expansion and excellent transparency, and a porcelain comprising a powder of the glass ceramic.
A further object of the invention is to provide a leucite crystal-containing glass-ceramic porcelain in which the leucite crystals, once precipitated, do not substantially decrease in amount during the process of coating a metal frame.
A further object of the invention is to provide a porcelain in which crystals of types other than leucite crystals (e.g., Naxe2x80x94K feldspathic crystals) do not substantially precipitate during the process of coating a metal frame, in other words, to provide a leucite crystal-containing glass-ceramic porcelain in which crystals of types other than leucite crystals (e.g., Naxe2x80x94K feldspathic crystals) begin to precipitate sufficiently long after the precipitation of leucite crystals reaches the saturation point.
A further object of the invention is to provide a leucite crystal-containing glass-ceramic porcelain which is free of opacification and decrease in the coefficient of thermal expansion during the process of coating a metal frame with the porcelain.
A further object of the invention is to provide a dental prosthesis obtainable by building up and fusion-bonding the leucite crystal-containing glass-ceramic porcelain onto the surface of a metal frame.
The above objects of the invention can be achieved by mixing a glassy material and leucite crystals (seed crystals) previously synthesized, and then heat-treating the resulting mixture under specific conditions.
Specifically, the present invention provides a method for preparing a leucite crystal-containing glass-ceramic, comprising the steps of:
mixing
(1) a glassy material comprising 53 to 65 wt. % of SiO2, 13 to 23 wt. % of Al2O3, 9 to 20 wt. % of K2O and 6 to 12 wt. % of Na2O, and
(2) synthetic leucite seed crystals comprising 53 to 64 wt. % of SiO2, 19 to 27 wt. % of Al2O3 and 17 to 25 wt. % of K2O;
and heat-treating the mixture at 750 to 950xc2x0 C. for 1 to 5 hours.
The glassy material (1) for use in the invention comprises SiO2, Al2O3, K2O and Na2O as essential components. Na2O is a component which lowers the fusing point of the glassy material (1).
The glassy material (1) may contain optional components which does not prevent precipitation of leucite crystals or inhibit the transparency of the glass-ceramic, such as F and colorless oxides of Li, Mg, Ca, Sr, B, P, Ti, Zr, etc. Specific examples of optional components include Li2O (2 wt. % or less), MgO (3 wt. % or less), CaO (3 wt. % or less), SrO (2 wt. % or less), B2O3 (3 wt. % or less), P2O5 (2 wt. % or less), TiO2 (3 wt. % or less), ZrO2(1 wt. % or less) and F (2 wt. % or less). It is preferable that the total proportion of these optional component(s) in the glassy material (1) be 6 wt. % or less.
The concomitant use of the optional components accomplishes the following effects. Oxides of Li, Mg, Ca, Sr, B, P, Ti or the like are effective for lowering the fusing point of the glass-ceramic. Oxides of Mg, Ca, Sr, B, Ti or the like improve the water resistance and acid resistance of the glass-ceramic. Oxides of Mg, Ca, Sr, Ti, Zr or the like improve the alkali resistance of the glass-ceramic.
In this specification and the appended claims, when an expression such as, for example, xe2x80x9ccomprising 2 wt. % or lessxe2x80x9d, is used to indicate an oxide content, it includes the case where no oxide is contained.
More preferably, the glassy material (1) comprises about 61 to 65 wt. % of SiO2, about 12 to 20 wt. % of Al2O3, about 10 to 15 wt. % of K2O, about 6 to 10 wt. % of Na2O, 0.3 wt. % or less of Li2O, 1.0 wt. % or less of MgO, 2 wt. % or less of CaO, about 0.3 to 1.5 wt. % of B2O3, 1 wt. % or less of SrO, 2 wt. % or less of TiO2, 0.5 wt. or less of ZrO2, 0.5 wt. % or less of P2O5 and 1.5 wt. % or less of F. In the glassy material having the above composition, the total proportion of the optional components is preferably 6 wt. % or less.
The glassy material (1) can be prepared by a known melting process, for example by melting a starting mixture comprising predetermined proportions of components such as oxides, hydroxides, carbonates or the like, at about 1550 to 1750xc2x0 C. for about 2 to 5 hours, more preferably at 1600 to 1700xc2x0 C. for about 3 to 4 hours. The starting mixture can be melted in a conventional crucible such as a high alumina crucible, a platinum crucible, an Rh-containing platinum crucible, a Zr-containing platinum crucible or the like. Among them, an Rh-containing platinum crucible and Zr-containing platinum crucible are more preferable.
In producing the glass-ceramic, the glassy material (1) is preferably used in powder form. A powder of the glassy material (1) can be prepared, for example, in the following manner: a molten glass obtained by the above melting process is poured into water, or a crucible containing the molten glass is placed in water, to suddenly cool and coarsely crush the molten glass. The glassy portion is then separated, collected, dried and pulverized to a predetermined size (a particle size distribution with usually 200 mesh or smaller (=about 75 um or smaller), more preferably a mean particle size of about 30 to 60 xcexcm), using a roll mill, ball mill, jet mill or like pulverizer, optionally followed by sieving.
In the invention, the synthetic leucite seed crystals (2) for use as seed crystals may be synthetic leucite crystals having a theoretical composition, a leucite solid solution containing SiO2 dissolved therein, synthetic leucite crystals in which a part (5% or less) of K is substituted by Rb, or a mixture of at least two of these types of leucite crystals. When the synthetic leucite seed crystals (2) are used in a mixture form, the mixing ratio of each component is not limited. Preferably, synthetic leucite crystals having a theoretical composition are used as seed crystals.
The synthetic leucite seed crystals (2) for use in the invention comprise SiO2, Al2O3 and K2O as essential components. The synthetic leucite seed crystals (2) may contain optional component(s) which neither decrease(s) the leucite crystal content to 80 wt. % or less, nor reduce(s) the transparency. Such optional components include F and oxides of Li, Na, Mg, Ca, Sr, B, P, Ti, Zr, etc. These optional components reduce a crystallinity in the seed crystals and thus cannot be used in large proportions, but promote diffusion of the essential components and achieve a lower melting point of the essential components while melting. A preferable total content of the optional component(s) in the synthetic leucite crystal seed (2) is 3 wt. % or less.
More preferably, the synthetic leucite seed crystals (2) comprise about 53 to 56 wt. % of SiO2, about 22 to 25 wt. % of Al2O3 and about 20 to 25 wt. % of K2O.
The synthetic leucite seed crystals (2) are not limited, but may be prepared by a conventional melting process similar to the process for preparing the glassy material (1). For example, a starting mixture comprising predetermined proportions of oxides, hydroxides, carbonates or the like is melted at 1700xc2x0 C. or higher (more preferably about 1750xc2x0 C.) for at least 2 hours (more preferably about 3 hours), slowly cooled to about 1300xc2x0 C. at a cooling rate of about 100xc2x0 C./hr or slower (more preferably about 50xc2x0 C./hr) to complete crystallization, and then allowed to cool to room temperature (about 15 to 25xc2x0 C.), thereby giving synthetic leucite crystals. The formation of synthetic leucite crystals can be easily confirmed by powder X-ray diffraction analysis of the product. In the above slow cooling process, it is preferable that, at an intermediate stage of the slow cooling, the melt is maintained for example at 1400 to 1500xc2x0 C. for about 2 to 3 hours. The starting mixture can be melted in a conventional crucible such as a high alumina crucible, a platinum crucible, an Rh-containing platinum crucible, a Zr-containing platinum crucible or the like.
Alternatively, the synthetic leucite seed crystals (2) can be synthesized by placing the above starting mixture in a crucible and maintaining the mixture at a temperature not lower than 1400xc2x0 C. for a given period of time for firing. In this case, the higher the firing temperature is, the shorter time the synthesis requires. For example, when the firing temperature is about 1600xc2x0 C., the mixture is maintained for about 5 hours to obtain seed crystals with a crystallinity of about 95%. In contrast, when the firing temperature is about 1400xc2x0 C., the mixture needs to be maintained for about 3 to 6 days to obtain seed crystals having the same degree of crystallinity as above.
Preferably, in production of the glass-ceramic, the synthetic leucite seed crystals (2) are also used in pulverized form. A powder of the synthetic leucite seed crystals (2) can be obtained for example as follows: the crucible containing the high-temperature synthetic leucite crystals prepared in the above manner is placed in water to suddenly cool and coarsely crush the crystals, and the crystals are separated, collected, dried and then pulverized to a predetermined size (usually a particle size distribution with 200 mesh or smaller (=about 75 xcexcm or smaller), more preferably a mean particle size of about 30 to 60 xcexcm) using a pulverizer such as a roll mill, ball mill, jet mill or the like, optionally followed by sieving. Note that the synthetic leucite seed crystals (2) may be of any size and are not limited to the above sizes, as long as they function as seed crystals. For example, the use of leucite crystals in the form of fine powder with a mean particle size of about 3.5 xcexcm achieves desired effects.
In producing the glass-ceramic of the invention, a starting mixture is used which comprises, per 100 parts by weight of a powder of the glassy material (1), 0.5 to 3 parts by weight of a powder of the leucite seed crystals (2) with a high purity synthesized in the above manner. The use of an excess amount of the leucite seed crystals (2) leads to opacification of the glass-ceramic, and thus is undesirable.
The proportion of the synthetic leucite seed crystals (2) to the glassy material (1) is more preferably 1 to 2 parts by weight per 100 parts by weight of the glassy material (1). When using these ingredients in such proportions, a glass-ceramic can be obtained which has further improved characteristics.
The glass-ceramic of the invention can be prepared by heat-treating the above mixture usually at about 750 to 950xc2x0 C. for about 1 to 5 hours, more preferably at about 800 to 900xc2x0 C. for about 3 to 5 hours.
When the glass-ceramic of the invention is used as, for example, a porcelain, the glass-ceramic is made into a powder by a conventional pulverizing process, or into a controlled particle size powder by sieving the powder. The powder for use as a porcelain is not limited, but usually has a particle size of 100 xcexcm or smaller. More preferably, the powder has a mean particle size of 5 to 50 xcexcm and contains 1% or less of fine particles with a particle size of 1 xcexcm or smaller.
If the glassy material (1) alone (without the synthetic leucite seed crystals) is heat-treated, the results are as follows: Even when the glassy material (1) alone is heat-treated, leucite crystals form and slowly grow. This is presumably because the edges of the pulverized glassy material, or trace amounts of high-melting inorganic impurities (fine particles of Al2O3, SiO2 or the like mixed in during pulverization) serve as crystal nuclei. However, when the leucite crystal growth depends on such crystal nuclei formed spontaneously, a long period of time is required for the leucite crystals in the glass phase to grow and reach the saturation point. At the time when the leucite crystals reach the saturation point, the glass-ceramic having a desired coefficient of thermal expansion is formed. At that time, however, different types of crystals such as Naxe2x80x94K feldspathic crystals (stable phases) have begun to precipitate. Therefore, it is impossible to effectively prevent the decrease in the coefficient of thermal expansion (i.e., opacification) of the glass-ceramic.
On the other hand, the starting mixture for use in the invention contains a specific amount of synthetic leucite seed crystals (2), so that, in the glass phase under heated conditions, leucite crystals rapidly grow and reach the saturation point in a short time. Accordingly, when the resulting glass-ceramic is used as a porcelain, there is a sufficiently long period of time before different types of crystals such as Naxe2x80x94K feldspathic crystals begin to precipitate. As a result, even when the glass-ceramic is used as a porcelain under high temperature conditions, the leucite crystals remain stable and the opacification owing to precipitation of different types of crystals (feldspars) is effectively inhibited.
An excessive proportion of the synthetic leucite seed crystals (2) in the starting mixture used in the invention causes the following problems: When the powder mixture of the glassy material (1) and leucite crystals (2) is heat-treated for sinter-crystallization, the glassy material (1) powder and seed crystal (2) powder are not fully fusion-bonded to each other and leave minute voids, leading to opacification. Moreover, when the glass-ceramic porcelain powder is fusion-bonded onto a metal frame, it is difficult to degas the porcelain, resulting in formation of fine cracks in the fusion-bonded object, which is not practical.
The leucite crystals in the glass-ceramic obtained by the method according to the invention have a particle size up to 200 mesh (up to about 75 xcexcm). It is thus apparent that leucite crystals derived from the seed crystals (2) maintain their original particle size. On the other hand, leucite crystals which have grown using the seed crystals (2) as the crystal origin point have a mean particle size of about 5 xcexcm or smaller, mainly because (a) they grow from a number of pieces of crystal origin point, and (b) the crystal growth rate is low since the heat treatment for crystal growth is carried out at a low temperature of 750 to 950xc2x0 C. Accordingly, there is a state that creates a tensile stress between the leucite crystal phase (coefficient of thermal expansion xcex1=about 22xc3x9710xe2x88x926/xc2x0 C.) and the matrix glass phase (xcex1=about 9xc3x9710xe2x88x926/xc2x0 C.) in the glass-ceramic obtained by crystallization treatment, owing to the difference between the coefficients of thermal expansion of the two phases. When this ceramic-glass is pulverized, portions with larger tensile stress, i.e., portions in which the particle size of leucite crystals is larger, are selectively crushed. As a result, even when the glass-ceramic porcelain powder has a particle size up to 200 mesh (up to about 75 xcexcm), the leucite crystal particles with a large particle size derived from the seed crystals (2) are remarkably decreased.
The glass-ceramic obtained by the invention contains leucite crystals, and has a coefficient of thermal expansion of (12 to 17.5)xc3x9710xe2x88x926/xc2x0 C. at 50 to 500xc2x0 C. The glass-ceramic of the invention has the characteristic that, even if heat-treated under severe conditions (at 850xc2x0 C. for 3 hours or at 750xc2x0 C. for 10 hours), the glass-ceramic is substantially free of reduction in leucite crystal content or precipitation of different types of crystals such as Naxe2x80x94K feldspathic crystals.
The leucite crystals in the glass-ceramic obtained by the method of the invention may be leucite crystals having a theoretical composition, a leucite solid solution containing a SiO2 component dissolved therein, leucite crystals in which a part (2% or less) of K is substituted by Rb, or a mixture containing at least two of these types of crystals.
In the glass-ceramic obtained by the invention, leucite crystals in an amount equal or close to the saturation amount at the crystal growth temperature during the production of the glass ceramic are uniformly dispersed as a metastable crystal phase having a mean particle size of 10 xcexcm or smaller (more preferably 5 xcexcm or smaller).
The leucite crystal content in the glass-ceramic according to the invention is about 15 to 43 wt. %, although depending on the proportions of the glassy material (1) and the synthetic leucite seed crystals (2), heat treatment conditions in producing the glass-ceramic, or other factors.
When a powder or a controlled particle size powder of the glass-ceramic according to the invention is used as a porcelain, any known additives such as opacifiers, coloring pigments, fluorescent materials or the like may be added as required, as long as they do not inhibit the effects of the invention.
Useful opacifiers include, for example, rutile or anatase TiO2, SnO2, ZrSiO4, CeO2, and stabilized ZrO2 (stabilizer=Y2O3, CaO, MgO or the like).
Examples of coloring pigments include Fe2O3 pigments, Fe2O3xe2x80x94Cr2O3 pigments, Fe2O3xe2x80x94CoOxe2x80x94Cr2O3 pigments, PrO2 pigments, V2O5 pigments, CeO pigments, MnO2 pigments and SnO2xe2x80x94Cr2O3 pigments.
Examples of fluorescent materials include Ce-doped Y2O3.
The porcelain mixture containing specific additives is kneaded in a routine manner using water, modeling liquid (e.g., aqueous PVA solution), and then applied to or built up on a metal frame. To improve the workability, a known paste-form kneading agent (such as polyethylene glycol dimethyl ether, polyethylene glycol with a specific degree of polymerization, or the like) may be added to make the porcelain mixture into a paste.
When the glass-ceramic powder according to the invention is used as a porcelain for dental prosthesis fabrication, the powder can be used by the same method as for known porcelains. For example, a coating process consisting of building up, fusion-bonding and cooling the porcelain on the surface of a metal frame is repeated several times to form the external shape of dental prosthesis. The glass-ceramic according to the invention undergoes no substantial change in leucite crystal content when fired and thus has a substantially constant coefficient of thermal expansion, even after the glass-ceramic is subjected to 10 cycles, each consisting of (i) vacuum firing defined in JIS T 6515 (manufacturer""s specific process) and (ii) placing the glass-ceramic in a furnace at a predetermined temperature (for example 600xc2x0 C.), vacuumizing the furnace, then raising the temperature in the furnace to 900xc2x0 C. at a rate of 60xc2x0 C./min, and taking out the glass-ceramic and allowing it to cool in the atmosphere (total firing time=about 1 hour). Further, the fired body obtained in the above manner is highly transparent since precipitation of different types of crystals such as Naxe2x80x94K feldspathic crystals is inhibited. Therefore, the glass-ceramic obtained according to the invention is extremely useful as a porcelain for use on a dental metal frame.
Preferably, the metal frame is made of a known precious metal alloy. Examples of precious metal alloys include high karat golds, medium karat golds, gold-silver-palladium alloys, gold-palladium alloys and silver-palladium alloys.
In building up and fusion-bonding a powder of the glass-ceramic of the invention onto the metal frame, an undercoat porcelain is used to conceal the metal color. As the undercoat porcelain, a glass-ceramic powder containing an opacifier (alias xe2x80x9copaque porcelainxe2x80x9d) or a kneaded paste thereof (alias xe2x80x9cpaste opaquexe2x80x9d) is preferably used. As a topcoat porcelain formed to imitate natural teeth (alias xe2x80x9cdentinexe2x80x9d or xe2x80x9cenamelxe2x80x9d), a glass-ceramic powder containing a coloring pigment (which may optionally contain a small amount of opacifiers) is preferably used.